1. Field of the Invention
The present invention relates to a method for determining the thickness of a coating on a substrate, particularly on a metal substrate, and to coating compositions adapted for use in such a method.
2. Description of the related art
It is known to pre-treat metal substrates, particularly zinc, aluminum and their alloys with chromium-containing compositions to inhibit corrosion and promote adhesion of subsequently applied coatings. While effective, there are several disadvantages to using chromium treatments for this purpose. First, chromium treatments can cause yellow or blue discoloration of the substrate. Additionally, darkening of the substrate is occasionally observed after the chromium-treated substrate has been post-oiled for forming or lubrication. Also, once the metal substrate is chromium treated, no further post-treatment of the substrate, such as zinc phosphating, can be performed. This makes chromium treated metals unsuitable for use in coil coating and in automotive applications. Lastly, chromium is undesirable because of toxicity and waste disposal concerns.
Alternatives to chromium treatment have been described. Metal can be passivated with inorganic phosphate. Alternatively, the metal can be pre-treated with a curable or dryable film-forming organophosphates. For instance, vinyl polymers, i.e. acrylic polymers, and epoxy compounds having pendant phosphate groups can be applied to metal to provide a protective coating. Suitable curable film-forming organophosphate coatings are those of U.S. Pat. No. 5,294,265, which is incorporated herein by reference. These coatings are known by their commercial designation NUPAL(copyright). These coatings are commonly prepared from the reaction product of a phosphate and an epoxide.
These organophosphate coatings are, typically, deposited onto the metal substrate by rolling, spraying or dipping. The coatings are typically very thin and difficult to measure without time- and resource-consuming titration experiments. For example, the coatings can range from less than about 55 mg per m2 (5 mg per ft2) to over 430 mg per m2 (40 mg per ft2). Coatings of this range of thickness, especially coatings of less than 54 mg per m2 ( 5 mg per ft2), are typically difficult to measure with accuracy and precision by standard titration methods. Further, titration methods for determining coating thickness, such as the cerium titration method, are invasive, destructive to the metal and take significant time to complete. It is therefore desirable to have a non-invasive method by which the thickness of coatings, especially of very thin coatings, can be determined rapidly and automatically with precision and accuracy, e.g. on a moving strip in a rolling mill.
Methods for determining the presence of thin films on a substrate are known. For instance, U.S. Pat. No. 5,516,696 discloses using certain fluorescent dyes in metal film coatings to determine whether or not the coating has been applied to the surface. Specifically, the fluorescent brightening agents stilbene and coumarin are added to a chrome-free metal coating and, after coating the metal with the coating, the metal is viewed under ultraviolet (UV) light and the presence of coating is detected by eye. Although useful in its ability to determine whether a coating has been applied to a substrate, the methods described in that patent do not involve quantitative determination of the thickness of the coating, an important quality control matter. Although possible, use of stilbene and coumarin in quantitative determinations of thickness of a coating is not preferred because it has been found that these compounds often do not display the required precision in their use in a calibrated system to determine coating thickness that is required in a commercially feasible coating thickness measurement system. The native fluorescence of the coating can interfere with measurement of fluorescent intensity of the coating. Further, when these compounds are used according to the prior art patent, they can be extracted during subsequent processing with liquid surface contact or migrate into additional coating layers which are applied onto the coating.
Methods for determining thickness of transparent oil films on metal surfaces by detection of fluorescent compounds mixed in the oil are also known. However, due to the nature of oil films, these methods are not precise and, therefore, are not suitably reproducible for determination of the thickness of a dryable, dried, curable or cured film-forming coating on a substrate. Further, a method is needed to determine the thickness of films that are not as transparent as oil. Choice of dye is not important when the dye is used to measure oil film thickness. The layer of oil is typically not maintained on the surface of the substrate if additional layers of a coating are needed on the oil-covered substrate, such as a pre-coating a primer or a color coat. Since the oil layer is removed, there are no considerations of the effect of dye migration. Lastly, the dyes used to determine the oil film thickness show strong substrate dependence; i.e. coumarin or 4-(dicyanomethylene)-2-methyl-6-(4-dimethyl-aminostyryl)-4H-pyran, CAS No. 51325-91-8 (xe2x80x9cDCMxe2x80x9d).
The present invention provides a method for precisely and accurately measuring the thickness of a coating on a substrate. The coating is prepared from a curable or dryable film-forming coating composition, preferably including an organic resin and a fluorescent component. The fluorescent component is either an intrinsic fluorescence of an organic resin in the coating or a fluorescent dye distributed within the coating or covalently linked to an organic resin. A mathematical relationship is determined by a calibration procedure which relates the fluorescence intensity of the coating when exposed to a light source to the coating thickness. The mathematical relationship is then used to determine the thickness of the coating on a test sample. The thickness of the coating on the test sample is determined by exposing the coating to light of a wavelength suitable to cause the fluorescent component of the coating to fluoresce and determining the thickness of the coating on the test substrate based upon the pre-determined mathematical relationship. Surprisingly, it has been found that the coating does not need to be entirely transparent and the method is not substrate dependent.
The present invention is also directed to a facility for performing the method of the present invention. The facility includes, minimally, a detection station. Prior to reaching the detection station, a test substrate is coated with a curable or dryable film-forming coating containing a fluorescent component. At the detection station, the coating is exposed to light from a light source of a wavelength sufficient to cause the fluorescent component of the coating to fluoresce. The intensity of the fluorescence of the coating is detected by a detector that is capable of producing an analog or digital signal, which indicates the intensity of the fluorescent emission of the coating. Preferably, the detector is connected to a computing device which receives the analog or digital signal of the detector and converts the signal to a reading of the thickness of the coating by converting the signal to a measurement of the intensity of the fluorescent emission of the coating. The computing device then calculates the thickness of the coating on the substrate by entering the measurement of the intensity of the fluorescent emission of the coating into a predetermined mathematical relationship between the thickness of the coating and the intensity of fluorescent emissions from the coating when exposed to the light. The station where the substrate is coated and the detecting station need not be on the same manufacturing line, or even in the same location.
The present invention also includes a curable or dryable film-forming coating composition which includes a curable or dryable film-forming organic resin having a fluorescent tag attached thereto. Preferably, the tag is a dye. Most preferably the dye is a fluorescein or a rhodamine.
Lastly, the present invention includes a coated substrate having a coating of the curable or dryable film-forming coating composition.